Light-emitting devices, in particular, light-emitting diodes (LEDs), are ubiquitous to modem display technology. More than 30 billion chips are produced each year and new applications, such as automobile lights and traffic signals, continue to grow. Conventional devices are made from inorganic compound semiconductors, typically AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue). These devices emit monochromatic light of a frequency corresponding to the band gap of the compound semiconductor used in the device. Thus, conventional LEDs cannot emit white light, or indeed, light of any “mixed” color, which is composed of a mixture of frequencies. Further, producing an LED even of a particular desired “pure” single-frequency color can be difficult, since excellent control of semiconductor chemistry is required.
Light-emitting devices of mixed colors, and particularly white LEDs, have many potential applications. Consumers would prefer white light in many displays currently having red or green light-emitting devices. White light-emitting devices could be used as light sources with existing color filter technology to produce full color displays. Moreover, the use of white LEDs could lead to lower cost and simpler fabrication than red-green-blue LED technology.
White LEDs are currently made by combining a blue LED with a yellow phosphor to produce white light. However, color control is poor with this technology, since the colors of the LED and the phosphor cannot be varied. This technology also cannot be used to produce light of other mixed colors.
It has been proposed to manufacture white or colored light-emitting devices by combining various derivatives of photoluminescent polymers such as poly(phenylene vinylene) (PPVs). One device that has been proposed involves a PPV coating over a blue GaN LED, where the light from the light-emitting device stimulates emission in the characteristic color of the PPV, so that the observed light is composed of a mixture of the characteristic colors of the device and the PPV. However, the maximum theoretical quantum yield for PPV-based devices is 25%, and the color control is often poor, since organic materials tend to fluoresce in rather wide spectra. Furthermore, PPVs are rather difficult to manufacture reliably, since they are degraded by light, oxygen, and water. Related approaches use blue GaN-based LEDs coated with a thin film of organic dyes, but efficiencies are low (see, for example, Guha et al. (1997) J. Appl. Phys. 82(8):4126-4128; III-Vs Review 10(1):4, 1997).
It has also been proposed to produce light-emitting devices of varying colors by the use of quantum dots (QDs). Mattoussi et al. (1998) J. Appl. Phys. 83:7965-7974; Nakamura et al. (1998) Electronics Lett. 34:2435-2436; Schlamp et al. (1997) J. Appl. Phys. 82:5837-5842; Colvin et al. (1994) Nature 370:354-357. Semiconductor nanocrystallites (i.e., QDs) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of QDs shift to the blue (higher energies) as the size of the QDs gets smaller. It has been found that a CdSe QD, for example, can emit light in any monochromatic color, in which the particular color characteristic of the light emitted is dependent only on the QD's size.
Currently available light-emitting diodes and related devices that incorporate quantum dots use QDs that have been grown epitaxially on a semiconductor layer. This fabrication technique is most suitable for the production of infrared light-emitting devices, but devices in higher-energy colors have not been achieved by this method. Further, the processing costs of epitaxial growth by currently available methods (molecular beam epitaxy and chemical vapor deposition) are quite high. Colloidal production of QDs is a much more inexpensive process, but QDs produced by this method have generally been found to exhibit low quantum efficiencies, and thus have not previously been considered suitable for incorporation into light-emitting devices.
A few proposals have been made for embedding colloidally produced QDs in an electrically conductive layer in order to take advantage of the electroluminescence of these QDs for a light-emitting device. Mattoussi et al. (1998), supra; Nakamura et al. (1998), supra; Schlamp et al. (1997), supra; Colvin et al. (1994), supra. However, such devices require a transparent, electrically conductive host matrix, which severely limits the available materials for producing devices by this method. Available host matrix materials are often themselves light-emitting, which may limit the achievable colors using this method.